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From Lin Ma <lin...@gmail.com>
Subject Re: how client location a region/tablet?
Date Thu, 23 Aug 2012 16:40:38 GMT
Thanks, Harsh!

- "HBase currently keeps a single META region (Doesn't split it). " -- does
it mean there is only one row in ROOT table, which points the only one META
region?
- In Big Table, it seems they have multiple META regions (tablets), is it
an advantage over HBase? :-)

regards,
Lin
On Thu, Aug 23, 2012 at 11:48 PM, Harsh J <harsh@cloudera.com> wrote:

> HBase currently keeps a single META region (Doesn't split it). ROOT
> holds META region location, and META has a few rows in it, a few of
> them for each table. See also the class MetaScanner.
>
> On Thu, Aug 23, 2012 at 9:00 PM, Lin Ma <linlma@gmail.com> wrote:
> > Dong,
> >
> > Some more thoughts, after reading data structure for HRegionInfo =>
> > http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HRegionInfo.html
> ,
> > start key and end key looks informative which we could leverage,
> >
> > - I am not sure if we could leverage this information (stored as part of
> > value in table ROOT) to find which META region may contains region server
> > information for row-key 123 of data table ABC;
> > - But I think unfortunately the information is stored in value of table
> > ROOT, other than key field of table ROOT, so that we have to iterate each
> > row in ROOT table one by one to figure out which META region server to
> > access.
> >
> > Not sure if I get the points. Please feel free to correct me.
> >
> > regards,
> > Lin
> >
> > On Thu, Aug 23, 2012 at 11:15 PM, Lin Ma <linlma@gmail.com> wrote:
> >
> >> Doug, very informative document. Thanks a lot!
> >>
> >> I read through it and have some thoughts,
> >>
> >> - Supposing at the beginning, client side cache for region information
> is
> >> empty, and the client wants to GET row-key 123 from table ABC;
> >> - The client will read from ROOT table at first. But unfortunately, ROOT
> >> table only contains region information for META table (please correct
> me if
> >> I am wrong), but not region information for real data table (e.g. table
> >> ABC);
> >> - Does the client have to call each META region server one by one, in
> >> order to find which META region contains information for region owner of
> >> row-key 123 of data table ABC?
> >>
> >> BTW: I think if there is a way to expose information about what range of
> >> table/region each META region contains from .META. region key, it will
> be
> >> better to save time to iterate META region server one by one. Please
> feel
> >> free to correct me if I am wrong.
> >>
> >> regards,
> >> Lin
> >>
> >>
> >> On Thu, Aug 23, 2012 at 8:21 PM, Doug Meil <
> doug.meil@explorysmedical.com>wrote:
> >>
> >>>
> >>> For further information about the catalog tables and
> region-regionserver
> >>> assignment, see thisŠ
> >>>
> >>> http://hbase.apache.org/book.html#arch.catalog
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> On 8/19/12 7:36 AM, "Lin Ma" <linlma@gmail.com> wrote:
> >>>
> >>> >Thank you Stack, especially for the smart 6 round trip guess for the
> >>> >puzzle. :-)
> >>> >
> >>> >1. "Yeah, we client cache's locations, not the data." -- does it mean
> for
> >>> >each client, it will cache all location information of a HBase
> cluster,
> >>> >i.e. which physical server owns which region? Supposing each region
> has
> >>> >128M bytes, for a big cluster (P-bytes level), total data size / 128M
> is
> >>> >not a trivial number, not sure if any overhead to client?
> >>> >2. A bit confused by what do you mean "not the data"? For the client
> >>> >cached
> >>> >location information, it should be the data in table METADATA, which
> is
> >>> >region / physical server mapping data. Why you say not data (do you
> mean
> >>> >real content in each region)?
> >>> >
> >>> >regards,
> >>> >Lin
> >>> >
> >>> >On Sun, Aug 19, 2012 at 12:40 PM, Stack <stack@duboce.net> wrote:
> >>> >
> >>> >> On Sat, Aug 18, 2012 at 2:13 AM, Lin Ma <linlma@gmail.com>
wrote:
> >>> >> > Hello guys,
> >>> >> >
> >>> >> > I am referencing the Big Table paper about how a client locates
a
> >>> >>tablet.
> >>> >> > In section 5.1 Tablet location, it is mentioned that client
will
> >>> cache
> >>> >> all
> >>> >> > tablet locations, I think it means client will cache root
tablet
> in
> >>> >> > METADATA table, and all other tablets in METADATA table (which
> means
> >>> >> client
> >>> >> > cache the whole METADATA table?). My question is, whether
HBase
> >>> >> implements
> >>> >> > in the same or similar way? My concern or confusion is, supposing
> >>> each
> >>> >> > tablet or region file is 128M bytes, it will be very huge
space
> (i.e.
> >>> >> > memory footprint) for each client to cache all tablets or
region
> >>> >>files of
> >>> >> > METADATA table. Is it doable or feasible in real HBase clusters?
> >>> >>Thanks.
> >>> >> >
> >>> >>
> >>> >> Yeah, we client cache's locations, not the data.
> >>> >>
> >>> >>
> >>> >> > BTW: another confusion from me is in the paper of Big Table
> section
> >>> >>5.1
> >>> >> > Tablet location, it is mentioned that "If the client¹s cache
is
> >>> stale,
> >>> >> the
> >>> >> > location algorithm could take up to six round-trips, because
stale
> >>> >>cache
> >>> >> > entries are only discovered upon misses (assuming that METADATA
> >>> >>tablets
> >>> >> do
> >>> >> > not move very frequently).", I do not know how the 6 times
round
> trip
> >>> >> time
> >>> >> > is calculated, if anyone could answer this puzzle, it will
be
> great.
> >>> >>:-)
> >>> >> >
> >>> >>
> >>> >> I'm not sure what the 6 is about either.  Here is a guesstimate:
> >>> >>
> >>> >> 1. Go to cached location for a server for a particular user region,
> >>> >> but server says that it does not have a region, the client location
> is
> >>> >> stale
> >>> >> 2. Go back to client cached meta region that holds user region
w/
> row
> >>> >> we want, but its location is stale.
> >>> >> 3. Go to root location, to find new location of meta, but the root
> >>> >> location has moved.... what the client has is stale
> >>> >> 4. Find new root location and do lookup of meta region location
> >>> >> 5. Go to meta region location to find new user region
> >>> >> 6. Go to server w/ user region
> >>> >>
> >>> >> St.Ack
> >>> >>
> >>>
> >>>
> >>>
> >>
>
>
>
> --
> Harsh J
>

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